Study of B0 (s) → K0 S h+h'- decays with first observation of B0 s → K0 S K± π± and B0 s → K0 S π+ π-

A search for charmless three-body decays of B0 and B 0s mesons with a K0 S meson in the final state is performed using the pp collision data, corresponding to an integrated luminosity of 1:0 fb-1, collected at a centre-of-mass energy of 7TeV recorded by the LHCb experiment. Branching fractions of the B0 (s) → K0 S h+h'- decay modes (h(') = π,K), relative to the well measured B 0 → K0 Sπ+π- decay, are obtained. First observation of the decay modes B0 s → K0S K±π ± and B0 s → K0 Sπ+ π- and confirmation of the decay B0 → K0 SK±π ± are reported. The following relative branching fraction measurements or limits are obtained B(B0 → K0 S K±π±)/B(B0 → K0 Sπ+ π-)=0:128 ± 0:017 (stat:) ± 0:009 (syst:) ; B(B0 → K0 S K+K-)/B(B0 → K0 Sπ+ π-)= 0:385 ± 0:031 (stat:) ± 0:023 (syst:) ; B(B0s → K0 Sπ+ π-)/B(B0 → K 0 Sπ+ π-)=0:29 ± 0:06 (stat:) ± 0:03 (syst:) ± 0:02 (fs=fd) ; B(B0 s → K0 sK±π ±)/B(B0 → K0Sπ +π-)= = 1:48 ± 0:12 (stat:) ± 0:08 (syst:) ± 0:12 (fs=fd) ; B(B0s → K0 SK+K-)/B(B0 → K 0Sπ+π-)ε [0:004; 0:068] at 90% CL.